1. Field of the Invention
The present invention relates to organic silicon-boron polymers which upon pyrolysis produce high-temperature ceramic materials. More particularly, it relates to the polyorganoborosilanes containing -Si-B-bonds which generate high-temperature ceramic materials (e.g., SiC, SiB.sub.4, SiB.sub.6, B.sub.4 C) upon thermal degradation. The processes for preparing these organic silicon-boron polymer precursors are also a part of this invention.
2. Description of the Prior Art
Silicon carbide, silicon nitride, silicon-boron-oxide and mixtures thereof have generated considerable interest as ceramic materials. They have high thermal and oxidation stability and, are among the hardest materials that can be made. Other advantageous properties include low electrical conductivity, low co-efficient of thermal expansion, thermal shock resistance, creep resistance, high strength at elevated temperatures, and corrosion resistance. A few references which are of general interest follow:
D. Seyferth et al. disclose in the Communications of the American Ceramic Society, July 1984, pp. C-132 to C-133, the high-yield synthesis of Si.sub.3 N.sub.4 /SiC/C ceramic materials by the pyrolysis of a polyoganosilazane. The polyorganosilazane is prepared by the reaction of dichlorosilane, H.sub.2 SiCl.sub.2, with gaseous ammonia. The ammonialysis product is pyrolyzed in nitrogen atmosphere to give a 70 percent yield of Si.sub.3 N.sub.4.
In a similar manner, D. Seyferth et al. disclose in U.S. Pat. No. 4,482,669 the preparation of pre-ceramic polymers having repeating units of --(NH--SiR).sub.n -- and ##STR3## where R is hydrogen or one of a variety of alkyl, aryl, alkylsilane, or alkylamino groups. Upon pyrolysis, these polymers are useful to produce varieties of silicon nitride or silicon nitride-silicon carbide. Silcon-boron polymers are not disclosed or suggested.
In U.S. Pat. No. 3,154,520, J. A. DuPont et al. disclose the preparation of monomeric high-energy boron-containing compounds of the formula: X--CH.sub.2 CH.sub.2 --Si(A).sub.y (R).sub.z, where X is --B.sub.5 H.sub.8 or --B.sub.10 H.sub.13, A is chloro or bromo and R is --CH.sub.3 or --CH.sub.2 CH.sub.3.
In U.S. Pat. No. 3,431,234, M. M. Fein et al. disclose novel polymeric products of a carboranylsilane prepared by reacting a compound of the formula: R--(CB.sub.10 H.sub.10 C)--(CH.sub.2).sub.a --Si(X).sub.2 --R' with water to form a polysiloxane (--Si--O--) polymer.
In U.S. Pat. No. 4,152,509, S. Yajima et al. disclose the preparation of a number of polysiloxanes by treating at least one boric acid composed with phenylsilane. Polyborosilanes are not disclosed. In U.S. Pat. No. 4,283,376, S. Yajima et al. disclose a process for producing polycarbosilane partly containing siloxane bonds (--Si--O--) wherein a part of the polycarbosilane comprises polyborosiloxane and a structure composed of boron, silicon, and oxygen. The polycarbosilane is converted into silicon carbide fibers by first preparing a spinning dope, treating the dope to render it infusible under tension or under no tension and firing the treated dope in a vacuum or in an inert atmosphere.
In U.S. Pat. No. 4,298,559, R. H. Baney et al. disclose the preparation of polysilanes having the average formula [(CH.sub.3).sub.2 Si][CH.sub.3 Si]. These polysilanes are prepolymers which produce silicon carbide ceramic materials when pyrolyzed in an inert atmosphere.
In U.S. Pat. No. 4,572,902, Matano et al. disclose a method for producing ceramics having a closed-pore size from a sintered body of a Si.sub.3 N.sub.4 --SiC composition. The process includes heating the ceramic sintered article in a flowing gas mixture of chlorine and nitrogen at 500.degree. C. to 1,500.degree. C.
In U.S. Pat. No. 4,490,192, A. Gupta et al. disclose the preparation of finely divided particles of less than 1 micron in diameter having the composition of B.sub.x Si.sub.y, B.sub.x N.sub.y, P.sub.x Si.sub.y, P.sub.x N.sub.y, and the like. These particles are prepared in flowing argon with pyrolysis occurring using a laser.
Takamiza et al. in Japanese patent publications Nos. 80500/78 and 101099/78 disclose the preparation of polymers made from methylchlorosilanes, however, no mention is made of the yields of ceramic material generated by the decomposition of the polysilanes. More recently in Japanese Kokai Nos. 79/114600 and 79/83098 suggest that the preparation of silicon carbide precursor polymers having a silicon-carbon (--Si--C--Si--) bond are prepared by heating organosilicon compounds including (CH.sub.3).sub.3 Si--Si(CH.sub.3).sub.2 Cl in the presence of B, Al, Si, Ge, Sn and Pb compounds or Hl and its salts at high temperatures.
Additional references of general interest in the art include B. G. Penn et al., Journal Applied Polymer Science, Vol 27, p. 3751, published in 1982; K. J. Wynne et al., Annual Reviews of Material Science, Vol. 14, p. 297, published in 1984; and H. Noth et al., Chem. Ber., Vol. 99, p. 2197, published in 1966.
None of the above references disclose or suggest the polymer precursors the polyorganoborosilanes or silicon-boron ceramic polymers of the present invention.
It is highly desirable to have polymer precursors for -Si-B-ceramic materials that are formed from readily available and relatively inexpensive starting materials and in high yield. Additional desirable properties include stability at room temperature for prolonged periods of time, relatively stable to hydrolysis at atmospheric moisture levels, and high yield of ceramic material upon pyrolysis.